Power storage device and method for manufacturing electrode

ABSTRACT

An electrode and a power storage device each of which achieves better charge-discharge cycle characteristics and is less likely to deteriorate owing to separation of an active material, or the like are manufactured. As the electrode for the power storage device, an electrode including a current collector and an active material layer that is over the current collector and includes a particle containing niobium oxide and a granular active material is used, whereby the charge-discharge cycle characteristics of the power storage device can be improved. Moreover, contact between the granular active material and the particle containing niobium oxide makes the granular active material physically fixed; accordingly, deterioration due to expansion and contraction of the active material which occur along with charge and discharge of the power storage device, such as powdering of the active material layer or its separation from the current collector, can be suppressed.

TECHNICAL FIELD

The present invention relates to a power storage device and a method for manufacturing an electrode.

Note that a power storage device in this specification refers to every element or device which has a function of storing electric power.

BACKGROUND ART

In recent years, a variety of power storage devices such as lithium secondary batteries, lithium-ion capacitors, and air cells have been developed. In particular, a lithium secondary battery in which charge and discharge are performed by transfer of lithium ions between a positive electrode and a negative electrode has been attracting attention as a secondary battery with high output and high energy density.

An electrode used for a power storage device is manufactured by forming an active material layer over one surface of a current collector. The active material layer is formed using an active material such as carbon or silicon, and ions serving as carriers can be inserted into and extracted from the active material. For example, when an active material layer is formed using silicon or phosphorus-doped silicon, the theoretical capacity is higher than that in the case where the active material layer is formed using carbon, which is advantageous in increasing the capacity of a power storage device (see Patent Document 1).

However, it is known that the volume of silicon serving as an active material is expanded when lithium ions are inserted into silicon and contracted when lithium ions are extracted from silicon. Therefore, a problem arises in that an active material layer is powdered and detached from a current collector along with charge and discharge, for example. As a result, the current collecting property in an electrode is decreased and the charge-discharge cycle characteristics are degraded. As a countermeasure against this, there is a method in which a surface of an active material layer is coated with carbon, copper, nickel, or the like to suppress powdering and separation of silicon. However, such coating may inhibit reaction between lithium and silicon and may reduce the charge-discharge capacity.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.     2001-210315

DISCLOSURE OF INVENTION

An object of one embodiment of the present invention is to provide a power storage device having improved performance such as better cycle characteristics and to provide a method for manufacturing an electrode for the power storage device.

One embodiment of the present invention is a power storage device which includes an electrode including a current collector and an active material layer that is over the current collector and includes a particle containing niobium oxide and a granular active material.

One embodiment of the present invention is a power storage device which includes a negative electrode including a current collector and an active material layer that is over the current collector and includes a particle containing niobium oxide and a granular active material, an electrolyte in contact with the negative electrode, and a positive electrode facing the negative electrode with the electrolyte interposed therebetween.

In the power storage device of one embodiment of the present invention, the active material contains one or more materials selected from silicon, tin, aluminum, and germanium. The above materials can be alloyed with lithium, and an impurity such as phosphorus or boron may be added to such a material to reduce electric resistance.

One embodiment of the present invention is a method for manufacturing an electrode, which includes the steps of applying, to a current collector, slurry including a binder, a conductive additive, a particle containing niobium oxide, and a granular active material; and performing baking. In the method, the particle containing niobium oxide is formed by a sol-gel method.

In the method for manufacturing an electrode of one embodiment of the present invention, the active material contains one or more materials selected from silicon, tin, aluminum, and germanium. The above materials can be alloyed with lithium, and an impurity such as phosphorus or boron may be added to such a material to reduce electric resistance.

The above active material forms an alloy with lithium, and lithium ions can be reversibly inserted into and extracted from the material. Silicon has a theoretical capacity of 4000 mAh/g, which is approximately ten times the theoretical capacity of graphite that is a commonly-used material, 372 mAh/g. However, as described above, the volume of silicon serving as an active material changes significantly owing to charge and discharge by insertion and extraction of lithium ions. Accordingly, a defect such as powdering of the active material layer or its separation from a current collector is caused along with charge and discharge, leading to serious deterioration in charge-discharge cycles.

In one embodiment of the present invention, contact between the granular active material and the particle containing niobium oxide makes the granular active material physically fixed; accordingly, deterioration due to expansion and contraction of the active material which occurs along with charge and discharge of the power storage device, such as powdering of the active material layer, can be suppressed. For this reason, it is preferable that the area where the particle containing niobium oxide is in contact with the granular active material be large. Niobium oxide is excellent in conducting ions serving as carriers (e.g., lithium ions) and does not inhibit a battery function even when niobium oxide covers an active material. However, if niobium oxide, which is an electric insulator, completely wraps the active material, the niobium oxide serves as a resistor between the current collector and the active material, which is unfavorable.

The active material and the niobium oxide have no limitation on their crystallinity and may be amorphous, microcrystalline, or single-crystal. In addition, a material having two or more kinds of crystallinity may be used.

Further, the particle containing niobium oxide may contain a lithium niobate such as Li₂Nb₂O₅.

The Li₂Nb₂O₅ is formed by reaction between Nb₂O₅ and Li in initial discharge of the battery. In charge and discharge thereafter, the Li₂Nb₂O₅ may be held, or Li may be desorbed from the Li₂Nb₂O₅ so that Nb₂O₅ is formed. Thus, the Li₂Nb₂O₅ formed on the active material functions as a stable inorganic solid electrolyte interface (SEI) instead of an organic SEI, thereby having effects of reduction in resistance, improvement in lithium diffusivity, suppression of volume expansion of the active material, and the like.

The electrode used for the power storage device is manufactured in such a manner that the active material layer including the particle containing niobium oxide and the granular active material is formed over the current collector, whereby the cycle characteristics of the power storage device can be improved.

In addition, the conductive additive or the binder, which is used for manufacture of the electrode, may include the particle containing niobium oxide.

As a material for the current collector, a material with high electric conductivity can be used, such as a metal element typified by platinum, aluminum, or copper. Alternatively, the current collector may be formed using a metal element that forms silicide by reacting with silicon.

In the power storage device, the electrolyte which is provided between the negative electrode and the positive electrode facing the negative electrode can be formed using a liquid or a solid, and the electrolyte may include the particle containing niobium oxide.

According to one embodiment of the present invention, in the electrode including the active material layer that is over the current collector and includes the particle containing niobium oxide and the granular active material, a carbon-based film can be provided over the active material layer.

The carbon-based film is formed of a membranous carbon-based material and includes graphite or 1 or more and 100 or less layers, preferably 10 or more and 30 or less layers, of graphene. When 2 to 3 layers of graphene are stacked, the thickness of the membranous carbon-based material is 1 nm to 2 nm. The membranous carbon-based material may be either amorphous or crystalline.

Such a membranous carbon-based material is advantageous in that it can serve as a conductive additive to form a wide conductive network.

According to one embodiment of the present invention, deterioration of a power storage device due to separation of an active material, or the like can be reduced. Thus, a power storage device having improved performance such as better cycle characteristics can be provided.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are a cross-sectional view and a top view, respectively, which illustrate an electrode of a power storage device of one embodiment of the present invention;

FIG. 2 is a chart showing a method for manufacturing an electrode of one embodiment of the present invention;

FIGS. 3A and 3B are a plan view and a cross-sectional view, respectively, which illustrate one embodiment of a power storage device;

FIGS. 4A to 4D are perspective views illustrating application examples of a power storage device;

FIG. 5 is a diagram illustrating an example of a structure of a wireless power feeding system;

FIG. 6 is a diagram illustrating an example of a structure of a wireless power feeding system;

FIG. 7 is a diagram illustrating a fabrication process of a power storage device; and

FIG. 8 is a graph showing measurement results of battery characteristics of power storage devices.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments and an example of the present invention will be described with reference to the drawings. Note that the present invention is not limited to the following description and it will be readily appreciated by those skilled in the art that modes and details of the present invention can be modified in various ways without departing from the spirit and scope thereof. Thus, the present invention should not be construed as being limited to the description of the following embodiments and example. In description using the drawings for reference, in some cases, the same reference numerals are used in common for the same portions in different drawings. Further, in some cases, the same hatching patterns are applied to similar portions, and the similar portions are not necessarily designated by reference numerals.

Embodiment 1

In this embodiment, an electrode of a power storage device which is one embodiment of the present invention and a method for manufacturing the electrode are described with reference to FIGS. 1A and 1B and FIG. 2.

FIGS. 1A and 1B illustrate one embodiment of an electrode of a power storage device. FIG. 1A is a cross-sectional view of the electrode, and FIG. 1B is a top view of the electrode. The electrode of the power storage device in FIGS. 1A and 1B includes a current collector 101, and an active material layer 100 which is provided over one surface of the current collector 101 and includes a particle 109 containing niobium oxide and a granular active material 103. Although not illustrated here for simplicity, a binder for fixing a particle, a conductive additive for improving electric conductivity, and a viscosity modifier (N-methyl-2-pyrrolidone: NMP) for adjusting viscosity may be included in the active material layer 100, in addition to the granular active material 103 and the particle 109 containing niobium oxide in FIGS. 1A and 1B.

The current collector 101 is formed as appropriate using a conductive material which can be used for a negative electrode current collector and has heat resistance high enough to withstand heat treatment performed later. Examples of the conductive material which can be used for the current collector include, but are not limited to, copper, platinum, aluminum, nickel, tungsten, molybdenum, titanium, and iron. Note that, in the case of using aluminum for the current collector, an aluminum alloy to which an element that improves heat resistance, such as silicon, titanium, neodymium, scandium, or molybdenum, is added is preferably used. Alternatively, an alloy of any of the above conductive materials may be used.

Alternatively, the current collector 101 may be formed using a metal element that forms silicide by reacting with silicon. Examples of the metal element that forms silicide by reacting with silicon include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel.

Alternatively, an oxide conductive material can be used for the current collector 101. Typical examples of the oxide conductive material include indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium tin oxide, indium zinc oxide, and indium tin oxide to which silicon oxide is added. Note that the current collector 101 may have a foil shape, a plate shape, or a net shape. With such a shape, the current collector 101 can hold its shape by itself, and a supporting substrate or the like is therefore not essential.

The granular active material 103 is preferably formed using a material that can be alloyed with an element whose ion gives or receives electric charge. The ion which gives or receives electric charge is an ion of an alkali metal such as lithium or sodium; an ion of an alkaline earth metal such as calcium, strontium, or barium; a beryllium ion; or a magnesium ion, and a lithium ion is preferably used. The granular active material 103 can be formed using, as a material that can be alloyed with lithium, one or more materials selected from silicon, tin, aluminum, and germanium, for example.

Slurry formed using the granular active material 103 and the like is applied to the current collector 101, and baking is performed; thus, an active material layer can be formed over the current collector. At this time, an active material with a small grain diameter is preferably used because the power storage capacity per unit volume can be increased.

The particle 109 containing niobium oxide can be formed by a sol-gel method, a solid phase method, or the like. Instead of using niobium oxide, an oxide of vanadium, tantalum, tungsten, zirconium, molybdenum, hafnium, chromium, or titanium can be used. In addition, the crystal structure of the particle 109 containing niobium oxide may be any of an amorphous structure, a polycrystalline structure, and a single-crystal structure.

<Method for Forming Particle Containing Niobium Oxide>

Here, a method for forming the particle 109 containing niobium oxide is described. First, ethyl acetoacetate serving as a stabilizer and toluene serving as a solvent are stirred into niobium alkoxide (Nb(OCH₂CH₃)₅), so that a solution is obtained. Then, niobium alkoxide in the solution gradually reacts with moisture in the air and is condensed into a gel through hydrolysis reaction expressed by Chemical Formula 1.

Nb(OCH₂CH₃)₅+5H₂O→Nb(OH)₅+5CH₂CH₃OH  (Chemical Formula 1)

The gel obtained through the above steps is diluted with water and stirred using an ultrasonic cleaner, so that a dispersion liquid is formed. Next, the dispersion liquid is subjected to baking (at approximately 500° C. to 600° C.); thus, a particle containing niobium oxide can be produced from the Nb(OH)₅ gel through condensation reaction expressed by Chemical Formula 2.

2Nb(OH)₅→(Nb(OH)₄)₂O+H₂O  (Chemical Formula 2)

In the above manner, the particle 109 containing niobium oxide can be formed.

Further, the granular active material 103, e.g., a silicon particle can be mixed into the gel produced through the hydrolysis reaction expressed by Chemical Formula 1. This enables the particles 109 containing niobium oxide to be uniformly attached to the granular active material 103.

<Method for Manufacturing Electrode>

Next, a method for manufacturing the electrode illustrated in FIGS. 1A and 1B is described with reference to FIG. 2.

First, as shown in Step S111 in FIG. 2, slurry is applied to a current collector. The slurry includes the granular active material 103 and the particle 109 containing niobium oxide. The thickness of the applied slurry is preferably, but not limited to, greater than or equal to 20 μm and less than or equal to 30 μm and may be adjusted as appropriate in accordance with desired battery characteristics.

By applying the slurry to the current collector, the active material layer 100 including the particle 109 containing niobium oxide and the granular active material 103 can be formed as illustrated in FIGS. 1A and 1B. The area where the particle 109 containing niobium oxide is in contact with a surface of the granular active material 103 is preferably as large as possible. Further, an alloy of niobium and the active material may be formed in a portion where the granular active material 103 and the particle 109 containing niobium oxide are in contact with each other.

<Method for Forming Slurry>

Here, a method for forming the slurry is described. The particle containing niobium oxide formed in the above manner by a sol-gel method or the like, the granular active material, a binder, a conductive additive, and a viscosity modifier are mixed and stirred; thus, slurry in which the particle containing niobium oxide is attached to a surface of the granular active material is formed. The slurry is prepared in such a manner that the conductive additive is dispersed in a solvent including the binder and then the active material is mixed therein. At this time, in order to improve the dispersion property, it is preferable to reduce the amount of the solvent so that a thick paste is obtained. After that, the solvent is added and the slurry is formed. The proportions of the particle containing niobium oxide, the active material, the conductive additive, the binder, and the solvent can be adjusted as appropriate; the higher the proportions of the conductive additive and the binder are, the higher the battery performance per amount of the active material can be.

Note that favorable charge-discharge capacity and cycle characteristics can be obtained when the grain diameter of the granular active material is small, and the grain diameter is preferably 10 μm or less, for example. Similarly, favorable charge-discharge capacity and cycle characteristics can be obtained when the grain diameter of the particle containing niobium oxide is small, and the grain diameter is preferably greater than or equal to 1 nm and less than or equal to 5 μm, more preferably greater than or equal to 1 nm and less than or equal to 500 nm.

As the conductive additive, a material which is itself an electron conductor and does not cause chemical reaction with other materials in a battery device is used. For example, carbon-based materials such as graphite, carbon fiber, carbon black, acetylene black, Ketjenblack, and VGCF (registered trademark); metal materials such as copper, nickel, aluminum, and silver; and powder, fiber, and the like of mixtures thereof can be given. The conductive additive is a material that assists electric conductivity between active materials; it is provided between active materials which are apart from each other and makes conduction between the active materials.

As examples of the binder, polyimide; polysaccharides such as starch, carboxymethyl cellulose, hydroxypropyl cellulose, regenerated cellulose, and diacetyl cellulose; vinyl polymers such as polyvinyl chloride, polyethylene, polypropylene, polyvinyl alcohol, polyvinyl pyrrolidone, polytetrafluoroethylene, polyvinylidene fluoride, ethylene-propylene-diene monomer (EPDM) rubber, sulfonated EPDM rubber, styrene-butadiene rubber, butadiene rubber, and fluorine rubber; and polyether such as polyethylene oxide can be given.

As the solvent, water, N-methyl-2pyrrolidone, lactic acid ester, or the like can be used.

Next, as shown in Step S112 in FIG. 2, heat treatment is performed to vaporize the solvent. The heat treatment may be performed using a hot plate, an oven, or the like at 350° C. in a nitrogen atmosphere, for example.

Next, as shown in Step S113, the slurry is preferably pressed downward with the use of a press machine or the like so that the adhesion between the current collector and the slurry is increased. Moreover, the active material layer is planarized by the pressing.

Next, as shown in Step S114, the current collector is punched out so that a desired shape is obtained.

Lastly, as shown in Step S115, heat treatment is performed again. The heat treatment may be performed at 100° C. in a reduced-pressure atmosphere, for example.

Through the above steps, the electrode illustrated in FIGS. 1A and 1B can be manufactured.

As described above, contact between the granular active material and the particle containing niobium oxide makes the granular active material physically fixed; accordingly, deterioration due to expansion and contraction of the active material which occur along with charge and discharge of the battery, such as powdering of the active material layer, can be suppressed.

Further, since the particle containing niobium oxide is formed in contact with the active material, an organic SEI can be prevented from being formed on the surface of the active material. Consequently, effects of reduction in resistance, improvement in lithium diffusivity, suppression of volume expansion of the active material, and the like can be achieved.

Thus, the electrode used for the power storage device is manufactured in such a manner that the active material layer including the particle containing niobium oxide and the granular active material is formed over the current collector, whereby the cycle characteristics of the power storage device can be improved.

Embodiment 2

In this embodiment, a structure of a power storage device is described with reference to FIGS. 3A and 3B.

First, a structure of a secondary battery is described below as a power storage device. Here, a structure of a lithium secondary battery that is a typical example of a secondary battery is described.

FIG. 3A is a plan view of a power storage device 151, and FIG. 3B is a cross-sectional view taken along dashed-dotted line A-B in FIG. 3A. In this embodiment, a sealed thin power storage device is described as the power storage device 151.

The power storage device 151 illustrated in FIG. 3A includes a power storage cell 155 in an exterior member 153. The power storage device 151 further includes terminal portions 157 and 159 which are connected to the power storage cell 155. For the exterior member 153, a laminate film, a polymer film, a metal film, a metal case, a plastic case, or the like can be used.

As illustrated in FIG. 3B, the power storage cell 155 includes a negative electrode 163, a positive electrode 165, a separator 167 provided between the negative electrode 163 and the positive electrode 165, and an electrolyte 169.

The negative electrode 163 includes a negative electrode current collector 171 and a negative electrode active material layer 173. Further, the negative electrode active material layer 173 is formed on one or both surfaces of the negative electrode current collector 171.

The positive electrode 165 includes a positive electrode current collector 175 and a positive electrode active material layer 177. Further, the positive electrode active material layer 177 is formed on one or both surfaces of the positive electrode current collector 175.

The negative electrode current collector 171 is connected to the terminal portion 159. The positive electrode current collector 175 is connected to the terminal portion 157. Further, the terminal portions 157 and 159 each partly extend outside the exterior member 153.

Note that, although the sealed thin power storage device is described as the power storage device 151 in this embodiment, the power storage device can have a variety of structures; for example, a button power storage device, a cylindrical power storage device, or a rectangular power storage device can be used. Further, although the structure where the positive electrode, the negative electrode, and the separator are stacked is described in this embodiment, a structure where the positive electrode, the negative electrode, and the separator are rolled may be employed.

As the negative electrode current collector 171, the current collector 101 described in Embodiment 1 can be used.

As the negative electrode active material layer 173, the active material layer 100 described in Embodiment 1 can be used. In addition, silicon, which has high capacity, is preferably used as the active material.

Aluminum, stainless steel, or the like is used for the positive electrode current collector 175. The positive electrode current collector 175 can have a foil shape, a plate shape, a net shape, a film shape, or the like as appropriate.

The positive electrode active material layer 177 can be formed using LiFeO₂, LiCoO₂, LiNiO₂, LiMn₂O₄, LiFePO₄, LiCoPO₄, LiNiPO₄, LiMnPO₄, V₂O₅, Cr₂O₅, MnO₂, or another lithium compound as a material. Note that, when carrier ions are alkali metal ions other than lithium ions or alkaline earth metal ions, the positive electrode active material layer 177 can be formed using, instead of lithium in the above lithium compounds, an alkali metal (e.g., sodium or potassium) or an alkaline earth metal (e.g., calcium, strontium, or barium).

As a solute of the electrolyte 169, a material containing lithium ions, i.e., carrier ions is used. Typical examples of the solute of the electrolyte include lithium salt such as LiClO₄, LiAsF₆, LiBF₄, LiPF₆, and Li(C₂F₅SO₂)₂N. Note that, when carrier ions are alkali metal ions other than lithium ions or alkaline earth metal ions, the solute of the electrolyte 169 can be formed using alkali metal salt such as sodium salt or potassium salt; alkaline earth metal salt such as calcium salt, strontium salt, or barium salt; beryllium salt; magnesium salt; or the like, as appropriate.

As a solvent of the electrolyte 169, a material which can transfer lithium ions (or other carrier ions) is used. As the solvent of the electrolyte 169, an aprotic organic solvent is preferably used. Typical examples of the aprotic organic solvent include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, acetonitrile, dimethoxyethane, and tetrahydrofuran, and one or more of these materials can be used. When a gelled polymer is used as the solvent of the electrolyte 169, safety against liquid leakage or the like is increased. In addition, the power storage device 151 can be thin and lightweight. Typical examples of the gelled polymer include a silicon gel, an acrylic gel, an acrylonitrile gel, polyethylene oxide, polypropylene oxide, and a fluorine-based polymer.

Alternatively, a solid electrolyte such as Li₃PO₄, Li₃PO(_(4-x))N_(x), or Li_(x)PS_(y) (x, y: natural number) can be used as the electrolyte 169. Further, the electrolyte 169 may contain niobium. The electrolyte 169 may contain vinylene carbonate or the like.

An insulating porous material is used for the separator 167. Typical examples of the separator 167 include cellulose (paper), polyethylene, polypropylene, and glass fiber. A single layer or a stack of any of these materials can be used.

A lithium secondary battery has small memory effect, high energy density, and high discharge capacity. In addition, the output voltage of the lithium secondary battery is high. For those reasons, the size and weight of the lithium secondary battery can be reduced. Further, the lithium secondary battery does not easily deteriorate owing to repetitive charge and discharge and can be used for a long time, leading to cost reduction.

Next, a capacitor is described as a power storage device. Typical examples of the capacitor include a double-layer capacitor and a lithium-ion capacitor.

In the case of a capacitor, the positive electrode active material layer 177 in the secondary battery illustrated in FIG. 3B is formed using a material into which lithium ions (or other carrier ions) and/or anions can be reversibly inserted. Typically, the positive electrode active material layer 177 can be formed using active carbon, a conductive polymer, or a polyacenic semiconductor (PAS), for example.

The lithium-ion capacitor has high charge-discharge efficiency which allows rapid charge and discharge, and has a long life to withstand repetitive use.

By using the electrode described in Embodiment 1 as the negative electrode 163, a power storage device having better cycle characteristics can be manufactured.

In addition, by using the current collector and the active material layer described in Embodiment 1 in a negative electrode of an air cell which is one embodiment of a power storage device, a power storage device having better cycle characteristics can be manufactured.

As described above, in one embodiment of the present invention, the current collector, and the active material layer including the particle containing niobium oxide and the granular active material can be used. Accordingly, a power storage device having better cycle characteristics can be provided. A power storage device which is less likely to deteriorate owing to separation of an active material, or the like can be provided.

Embodiment 3

In this embodiment, application examples of the power storage device described in Embodiment 2 are described with reference to FIGS. 4A to 4D.

The power storage device described in Embodiment 2 can be used in electronic devices, e.g., cameras such as digital cameras or video cameras, digital photo frames, mobile phones (also referred to as cellular phones or cellular phone devices), portable game machines, portable information terminals, and audio reproducing devices. Further, the power storage device can be used in electric propulsion vehicles such as electric cars, hybrid cars, train vehicles, maintenance vehicles, carts, and electric wheelchairs. Here, examples of the electric propulsion vehicles are described.

FIG. 4A illustrates a structure of a four-wheeled automobile 300 which is one of the electric propulsion vehicles. The automobile 300 is an electric car or a hybrid car. The automobile 300 is an example in which a power storage device 302 is provided in a bottom portion. In order to clearly show the position of the power storage device 302 in the automobile 300, FIG. 4B illustrates the outline of the automobile 300 and the power storage device 302 provided in the bottom portion of the automobile 300. The power storage device described in Embodiment 2 can be used as the power storage device 302. The power storage device 302 can be charged by external power supply using a plug-in technique or a wireless power feeding system.

FIG. 4C illustrates a structure of a motorboat 1301 which is one of the electric propulsion vehicles. FIG. 4C illustrates the case where a power storage device 1302 is provided on a side portion of the body of the motorboat 1301. The power storage device described in Embodiment 2 can be used as the power storage device 1302. The power storage device 1302 can be charged by external power supply using a plug-in technique or a wireless power feeding system. For example, a power feeding device for charging the motorboat 1301 (i.e., for charging the power storage device 1302) may be provided at a mooring in a harbor.

FIG. 4D illustrates a structure of an electric wheelchair 1311 which is one of the electric propulsion vehicles. FIG. 4D illustrates the case where the electric wheelchair 1311 includes a power storage device 1312 in a bottom portion. The power storage device described in Embodiment 2 can be used as the power storage device 1312. The power storage device 1312 can be charged by external power supply using a plug-in technique or a wireless power feeding system.

Embodiment 4

In this embodiment, an example in which a secondary battery that is an example of a power storage device of one embodiment of the present invention is used in a wireless power feeding system (hereinafter referred to as an RF power feeding system) is described with reference to block diagrams of FIG. 5 and FIG. 6. In each of the block diagrams, independent blocks show elements within a power receiving device and a power feeding device, which are classified according to their functions. However, it may be practically difficult to completely separate the elements according to their functions; in some cases, one element can involve a plurality of functions.

First, the RF power feeding system is described with reference to FIG. 5.

A power receiving device 600 is an electronic device or an electric propulsion vehicle which is driven by electric power supplied from a power feeding device 700, and can be applied to another device which is driven by electric power, as appropriate. Typical examples of the electronic device include cameras such as digital cameras or video cameras, digital photo frames, mobile phones (also referred to as cellular phones or cellular phone devices), portable game machines, portable information terminals, audio reproducing devices, display devices, and computers. Typical examples of the electric propulsion vehicle include electric cars, hybrid cars, train vehicles, maintenance vehicles, carts, and electric wheelchairs. In addition, the power feeding device 700 has a function of supplying electric power to the power receiving device 600.

In FIG. 5, the power receiving device 600 includes a power receiving device portion 601 and a power load portion 610. The power receiving device portion 601 includes at least a power receiving device antenna circuit 602, a signal processing circuit 603, and a secondary battery 604. The power feeding device 700 includes at least a power feeding device antenna circuit 701 and a signal processing circuit 702.

The power receiving device antenna circuit 602 has a function of receiving a signal transmitted by the power feeding device antenna circuit 701 or transmitting a signal to the power feeding device antenna circuit 701. The signal processing circuit 603 processes a signal received by the power receiving device antenna circuit 602 and controls charge of the secondary battery 604 and supply of electric power from the secondary battery 604 to the power load portion 610. In addition, the signal processing circuit 603 controls operation of the power receiving device antenna circuit 602. That is, the signal processing circuit 603 can control the intensity, frequency, or the like of a signal transmitted by the power receiving device antenna circuit 602. The power load portion 610 is a driving portion which receives electric power from the secondary battery 604 and drives the power receiving device 600. Typical examples of the power load portion 610 include a motor and a driver circuit. Another device which drives the power receiving device by receiving electric power can be used as the power load portion 610 as appropriate. The power feeding device antenna circuit 701 has a function of transmitting a signal to the power receiving device antenna circuit 602 or receiving a signal from the power receiving device antenna circuit 602. The signal processing circuit 702 processes a signal received by the power feeding device antenna circuit 701. In addition, the signal processing circuit 702 controls operation of the power feeding device antenna circuit 701. That is, the signal processing circuit 702 can control the intensity, frequency, or the like of a signal transmitted by the power feeding device antenna circuit 701.

The secondary battery of one embodiment of the present invention is used as the secondary battery 604 included in the power receiving device 600 in the RF power feeding system illustrated in FIG. 5.

With the use of the secondary battery of one embodiment of the present invention in the RF power feeding system, the discharge capacity or the charge capacity (also referred to as the amount of power storage) can be increased as compared with that of a conventional secondary battery. Therefore, the time interval of the wireless power feeding can be longer (frequent power feeding can be omitted).

In addition, with the use of the secondary battery of one embodiment of the present invention in the RF power feeding system, the power receiving device 600 can be formed to be compact and lightweight if the discharge capacity or charge capacity with which the power load portion 610 can be driven is the same as that of a conventional secondary battery. Therefore, the total cost can be reduced.

Next, another example of the RF power feeding system is described with reference to FIG. 6.

In FIG. 6, the power receiving device 600 includes the power receiving device portion 601 and the power load portion 610. The power receiving device portion 601 includes at least the power receiving device antenna circuit 602, the signal processing circuit 603, the secondary battery 604, a rectifier circuit 605, a modulation circuit 606, and a power supply circuit 607. The power feeding device 700 includes at least the power feeding device antenna circuit 701, the signal processing circuit 702, a rectifier circuit 703, a modulation circuit 704, a demodulation circuit 705, and an oscillator circuit 706.

The power receiving device antenna circuit 602 has a function of receiving a signal transmitted by the power feeding device antenna circuit 701 or transmitting a signal to the power feeding device antenna circuit 701. When the power receiving device antenna circuit 602 receives a signal transmitted by the power feeding device antenna circuit 701, the rectifier circuit 605 generates DC voltage from the signal received by the power receiving device antenna circuit 602. The signal processing circuit 603 has a function of processing a signal received by the power receiving device antenna circuit 602 and controlling charge of the secondary battery 604 and supply of electric power from the secondary battery 604 to the power supply circuit 607. The power supply circuit 607 has a function of converting voltage stored by the secondary battery 604 into voltage needed for the power load portion 610. The modulation circuit 606 is used when a certain response is transmitted from the power receiving device 600 to the power feeding device 700.

With the power supply circuit 607, electric power supplied to the power load portion 610 can be controlled. Thus, overvoltage application to the power load portion 610 can be suppressed, and deterioration or breakdown of the power receiving device 600 can be reduced.

In addition, with the modulation circuit 606, a signal can be transmitted from the power receiving device 600 to the power feeding device 700. Therefore, when it is judged from the amount of charge of the power receiving device 600 that a certain amount of electric power is stored, a signal is transmitted from the power receiving device 600 to the power feeding device 700 so that power feeding from the power feeding device 700 to the power receiving device 600 can be stopped. As a result, the secondary battery 604 is not fully charged, which increases the number of times the secondary battery 604 can be charged.

The power feeding device antenna circuit 701 has a function of transmitting a signal to the power receiving device antenna circuit 602 or receiving a signal from the power receiving device antenna circuit 602. When a signal is transmitted to the power receiving device antenna circuit 602, the signal processing circuit 702 generates a signal which is transmitted to the power receiving device. The oscillator circuit 706 is a circuit which generates a signal with a constant frequency. The modulation circuit 704 has a function of applying voltage to the power feeding device antenna circuit 701 in accordance with the signal generated by the signal processing circuit 702 and the signal with a constant frequency generated by the oscillator circuit 706. Thus, a signal is output from the power feeding device antenna circuit 701. On the other hand, when a signal is received from the power receiving device antenna circuit 602, the rectifier circuit 703 rectifies the received signal. From signals rectified by the rectifier circuit 703, the demodulation circuit 705 extracts a signal transmitted from the power receiving device 600 to the power feeding device 700. The signal processing circuit 702 has a function of analyzing the signal extracted by the demodulation circuit 705.

Note that any circuit may be provided between circuits as long as the RF power feeding can be performed. For example, after the power receiving device 600 receives a signal and the rectifier circuit 605 generates DC voltage, a circuit such as a DC-DC converter or regulator that is provided in a subsequent stage may generate constant voltage. Thus, overvoltage application to the inside of the power receiving device 600 can be suppressed.

The secondary battery of one embodiment of the present invention is used as the secondary battery 604 included in the power receiving device 600 in the RF power feeding system illustrated in FIG. 6.

With the use of the secondary battery of one embodiment of the present invention in the RF power feeding system, the discharge capacity or the charge capacity can be increased as compared with that of a conventional secondary battery; therefore, the time interval of the wireless power feeding can be longer (frequent power feeding can be omitted).

In addition, with the use of the secondary battery of one embodiment of the present invention in the RF power feeding system, the power receiving device 600 can be formed to be compact and lightweight if the discharge capacity or charge capacity with which the power load portion 610 can be driven is the same as that of a conventional secondary battery. Therefore, the total cost can be reduced.

Note that, in the case where the secondary battery of one embodiment of the present invention is used in the RF power feeding system and the power receiving device antenna circuit 602 and the secondary battery 604 overlap with each other, it is preferable that the impedance of the power receiving device antenna circuit 602 be not changed by deformation of the secondary battery 604 due to charge and discharge of the secondary battery 604 and deformation of an antenna due to the above deformation. When the impedance of the antenna is changed, in some cases, electric power is not supplied sufficiently. The secondary battery 604 may be placed in a battery pack formed of metal or ceramics, for example. Note that, in that case, the power receiving device antenna circuit 602 and the battery pack are preferably separated from each other by several tens of micrometers or more.

In this embodiment, the charging signal has no limitation on its frequency and may have any band of frequency as long as electric power can be transmitted. For example, the charging signal may have any of an LF band of 135 kHz (long wave), an HF band of 13.56 MHz (short wave), a UHF band of 900 MHz to 1 GHz (ultra high frequency wave), and a microwave band of 2.45 GHz.

A signal transmission method may be properly selected from various methods including an electromagnetic coupling method, an electromagnetic induction method, a resonance method, and a microwave method. In order to prevent energy loss due to foreign substances containing moisture, such as rain and mud, the electromagnetic induction method or resonance method using a low frequency band, specifically, very-low frequencies of 3 kHz to 30 kHz, low frequencies of 30 kHz to 300 kHz, medium frequencies of 300 kHz to 3 MHz, or high frequencies of 3 MHz to 30 MHz is preferably used.

This embodiment can be implemented in combination with any of the above embodiments.

Example 1

In this example, a secondary battery of one embodiment of the present invention is described. In this example, the secondary battery of one embodiment of the present invention and a secondary battery for comparison (hereinafter referred to as a comparative secondary battery) were fabricated and their characteristics were compared. A fabrication process of an electrode of the secondary battery is described below.

<Formation Process of Slurry>

First, a method for forming slurry used for the electrode of the secondary battery is described.

The slurry was formed by mixing silicon particles (with a grain diameter of approximately 5 μm) used as an active material, particles containing niobium oxide, Ketjenblack (KB) used as a conductive additive, and polyimide (PI) used as a binder. The weight ratio of the materials was as follows: (silicon particles+particles containing niobium oxide):KB:PI=80:5:15. In addition, the weight ratio of the silicon particles to the particles containing niobium oxide was 92:8.

It is preferable that the silicon particles be covered with niobium oxide; however, if the proportion of the area covered with niobium oxide, which has an insulating property, is too high, the resistance is increased. Therefore, the weight ratio of the active material to the sum of the active material and niobium oxide preferably falls within the range of 0.7 to 0.999.

Here, a method for forming the particle containing niobium oxide is described. First, 2 mol of ethyl acetoacetate serving as a stabilizer and toluene serving as a solvent were stirred into 1 mol of niobium alkoxide (Nb(OCH₂CH₃)₅), so that a solution was obtained. Then, niobium alkoxide in the solution gradually reacted with moisture in the air and was condensed into a gel through the hydrolysis reaction expressed by Chemical Formula 1. The gel obtained in this manner was diluted with water by approximately 70-fold in the weight ratio and stirred using an ultrasonic cleaner, so that a dispersion liquid was formed. Next, condensation reaction was caused by baking the dispersion liquid at 500° C.; thus, a particle containing niobium oxide (Nb₂O₅) was formed from the Nb(OH)₅ gel.

Through the above steps, the slurry was formed.

Next, an active material layer was formed by applying the slurry to a current collector, so that the electrode of the secondary battery was fabricated.

As a material for the current collector, titanium was used. As the current collector, a sheet of titanium foil (also referred to as a titanium sheet) with a thickness of 17 μm was used.

Next, heat treatment was performed at 350° C. in a nitrogen atmosphere for 3 hours so that the slurry was dried. After that, compression was performed with a pressing machine, and the current collector was punched out so that a desired shape was obtained.

Lastly, drying treatment was performed at 100° C. in a nitrogen atmosphere for 12 hours.

Through the above steps, the electrode of the secondary battery was fabricated.

<Fabrication Process of Secondary Battery>

A fabrication process of the secondary battery of this example is described.

With the use of the electrode whose active material layer was formed by applying the slurry to the current collector through the above steps, the secondary battery was fabricated. A method for fabricating the secondary battery is described below.

As illustrated in FIG. 7, the fabricated secondary battery includes a negative electrode 204, a positive electrode 232, a separator 210, an electrolyte solution (not illustrated), a housing 206, and a housing 244. In addition, the secondary battery includes a ring-shaped insulator 220. The negative electrode 204 was formed using lithium metal (lithium foil). The electrode formed through the above steps was used as the positive electrode 232. In this example, titanium foil was used as a current collector of the electrode, and an active material layer was formed by the method described in Embodiment 1; thus, the electrode was fabricated. The separator 210 and the ring-shaped insulator 220 were formed using polypropylene. The housing 206 and the housing 244 each of which was made of stainless steel (SUS) were used. The housing 206 and the housing 244 have a function of electrically connecting the negative electrode 204 and the positive electrode 232 to the outside.

The negative electrode 204, the positive electrode 232, and the separator 210 were soaked in the electrolyte solution. Then, as illustrated in FIG. 7, the housing 206, the negative electrode 204, the separator 210, the ring-shaped insulator 220, the positive electrode 232, and the housing 244 were stacked in this order with the bottom of the housing 206 at the lowest position and were fixed; thus, the secondary battery was fabricated.

An electrolyte solution in which LiPF₆ was dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) was used.

<Fabrication Process of Electrode of Comparative Secondary Battery>

Next, a fabrication process of an electrode of the comparative secondary battery is described. A difference between the secondary battery of one embodiment of the present invention and the comparative secondary battery lies in a fabrication process of a positive electrode; in a positive electrode of the comparative secondary battery, different slurry is applied to a current collector. The other components are common, and thus, description thereof is omitted.

The slurry for fabrication of the electrode of the comparative secondary battery was formed by mixing silicon particles (with a grain diameter of approximately 5 μm) used as an active material, Ketjenblack (KB) used as a conductive additive, and polyimide (PI) used as a binder. The weight ratio of the materials was as follows: silicon particles:KB:PI=80:5:15.

In the above manner, the active material layer was formed by applying the slurry to the current collector, so that an electrode serving as the positive electrode of the comparative secondary battery was fabricated. The comparative secondary battery was fabricated using the positive electrode. The comparative secondary battery was fabricated in the same manner as the above secondary battery except for the fabrication process of the positive electrode.

<Characteristics Comparison between Secondary Battery of the Present Invention and Comparative Secondary Battery>

Battery characteristics of the secondary battery and the comparative secondary battery which were fabricated in the above manners were compared using a charge-discharge measuring instrument. A constant current method was employed for the charge-discharge measurement. For the measurement, a current value corresponding to 0.5 C was used only in the initial discharge, and a current value at the time when the discharge capacity reached 2000 mAh/g in 2 hours was used in discharge thereafter. The measurement was performed at room temperature. Results are shown in FIG. 8.

FIG. 8 shows measurement results of the amount of released lithium ion with respect to the number of charge-discharge cycles. From these results, it is found that the lithium-ion discharge capacity of the secondary battery fabricated according to Embodiment 1 is not decreased even when the number of charge-discharge cycles is increased, in contrast to that of the comparative secondary battery.

The results in FIG. 8 indicate that the secondary battery fabricated according to Embodiment 1 has better cycle characteristics than the comparative secondary battery.

REFERENCE NUMERALS

-   100: active material layer, 101: current collector, 103: active     material, 109: particle, 151: power storage device, 153: exterior     member, 155: power storage cell, 157: terminal portion, 159:     terminal portion, 163: negative electrode, 165: positive electrode,     167: separator, 169: electrolyte, 171: negative electrode current     collector, 173: negative electrode active material layer, 175:     positive electrode current collector, 177: positive electrode active     material layer, 204: negative electrode, 206: housing, 210:     separator, 220: ring-shaped insulator, 232: positive electrode, 244:     housing, 300: automobile, 302: power storage device, 600: power     receiving device, 601: power receiving device portion, 602: power     receiving device antenna circuit, 603: signal processing circuit,     604: secondary battery, 605: rectifier circuit, 606: modulation     circuit, 607: power supply circuit, 610: power load portion, 700:     power feeding device, 701: power feeding device antenna circuit,     702: signal processing circuit, 703: rectifier circuit, 704:     modulation circuit, 705: demodulation circuit, 706: oscillator     circuit, 1301: motorboat, 1302: power storage device, 1311: electric     wheelchair, and 1312: power storage device.

This application is based on Japanese Patent Application serial no. 2011-186714 filed with the Japan Patent Office on Aug. 30, 2011, the entire contents of which are hereby incorporated by reference. 

1. A material being in contact with a particle comprising niobium oxide, wherein the material comprises one or more materials selected from silicon, tin, aluminum, and germanium.
 2. The material according to claim 1, wherein the particle comprises a lithium niobate.
 3. The material according to claim 2, wherein the lithium niobate is Li₂Nb₂O₅.
 4. The material according to claim 1, wherein a grain diameter of the particle is greater than or equal to 1 nm and less than or equal to 5 μm.
 5. The material according to claim 1, wherein a carbon-based film is provided over the material and the particle.
 6. The material according to claim 5, wherein the carbon-based film is formed of a membranous carbon-based material.
 7. The material according to claim 6, wherein the membranous carbon-based material is graphite or graphene.
 8. A power storage device comprising: an electrode comprising a current collector and an active material layer over the current collector, wherein the active material layer comprises a particle comprising niobium oxide and a granular active material.
 9. The power storage device according to claim 8, wherein the particle is in contact with the granular active material.
 10. The power storage device according to claim 8, wherein a grain diameter of the particle is greater than or equal to 1 nm and less than or equal to 5 μm.
 11. The power storage device according to claim 8, wherein the granular active material comprises one or more materials selected from silicon, tin, aluminum, and germanium.
 12. The power storage device according to claim 8, wherein the power storage device is a lithium secondary battery or a lithium-ion capacitor.
 13. A power storage device comprising: a negative electrode comprising a current collector and an active material layer over the current collector, wherein the active material layer comprises a particle comprising niobium oxide and a granular active material.
 14. The power storage device according to claim 13, wherein the particle is in contact with the granular active material.
 15. The power storage device according to claim 13, wherein a grain diameter of the particle is greater than or equal to 1 nm and less than or equal to 5 μm.
 16. The power storage device according to claim 13, wherein the granular active material comprises one or more materials selected from silicon, tin, aluminum, and germanium.
 17. The power storage device according to claim 13, wherein the power storage device is a lithium secondary battery or a lithium-ion capacitor.
 18. The power storage device according to claim 13, further comprising: an electrolyte in contact with the negative electrode; and a positive electrode facing the negative electrode with the electrolyte interposed therebetween.
 19. A method for manufacturing an electrode, comprising the steps of: applying a slurry to a current collector; and baking the slurry to form an active material layer, wherein the slurry comprises a binder, a conductive additive, a particle comprising niobium oxide, and a granular active material.
 20. The method for manufacturing an electrode according to claim 19, wherein the granular active material comprises one or more materials selected from silicon, tin, aluminum, and germanium.
 21. The method for manufacturing an electrode according to claim 19, wherein the particle is formed by a sol-gel method. 